1. Field of the Invention
The present invention relates to a substrate structure, and, more particularly, to a substrate structure having conductive bumps.
2. Description of Related Art
As electronic elements such as chips and packaging substrates have reduced trace widths and pitches, conductive pads of the electronic elements have narrower pitches therebetween. Generally, conductive bumps are disposed on the conductive pads and reflowed to form solder balls. However, the narrower pitches between the conductive pads likely cause solder bridging to occur between adjacent solder balls, thereby resulting in a short circuit.
To overcome the above-described drawback, metal posts having a high melting point (such as copper or nickel posts) are disposed on the conductive pads, followed by formation of a small amount of a solder material on the metal posts to serve as a bonding layer. Since the metal posts will not be melted into a ball shape during a reflow process, they are applicable to fine-pitch conductive pads.
Generally, the diameter of the conductive pads, the diameter of the metal posts and the amount of the solder material on the metal posts are uniform. Therefore, after the reflow process, the solder balls can have a uniform height. That is, the solder balls are coplanar, which facilitates to increase the bonding reliability between electronic elements.
However, along with the progress of technologies, some conductive pads of an electronic element are required to have a small diameter to meet the demand for more I/O counts, and some conductive pads of the electronic element are required to have a large diameter to facilitate electrical probe tests. Therefore, the conductive pads of different diameters are provided on the same electronic element. Moreover, in order to meet the coplanarity requirement, the height difference between conductive bumps that are disposed on the conductive pads of different diameters is required to be less than 8 μm.
As shown in FIG. 1A, a conventional packaging substrate 1 has a substrate body 10 having a plurality of conductive pads 100a, 100b, a plurality of first conductive bumps 11 disposed on the conductive pads 100a, and a plurality of second conductive bumps 12 disposed on the conductive pads 100b. Each of the first conductive bumps 11 has a first copper-nickel layer 110 and a first pre-solder layer 111 of tin-silver formed on the first copper-nickel layer 110. Each of the second conductive bumps 12 has a second copper-nickel layer 120 and a second pre-solder layer 121 of tin-silver formed on the second copper-nickel layer 120. The conductive pads 100a, 100b have different diameters. The width W of each of the second conductive bumps 12 is less than the width R of each of the first conductive bumps 11, but the height L of each of the second conductive bumps 12 with respect to the substrate body 10 is equal to the height L of each of the first conductive bumps 11 with respect to the substrate body 10.
To bond the packaging substrate 1 with a chip (not shown), the first and second pre-solder layers 111, 121 are reflowed into solder balls 111′, 121′, respectively, for the chip to be bonded thereto. During the reflow process, the first and second copper-nickel layers 110, 120 will not be melted into balls.
However, since the width R of each of the first conductive bumps 11 is different from the width W of each of the second conductive bumps 12, the height L′ of each of the first conductive bumps 11 with respect to the substrate body 10 is inconsistent with the height L″ of each of the second conductive bumps 12 with respect to the substrate body 10 after the reflow process is performed on the first and second pre-solder layers 111, 121 that have the same thickness, and the height difference is greater than 8 μm. Therefore, the coplanarity requirement is not met, as shown in FIG. 1B, and the height L″ of each of the second conductive bumps 12 with respect to the substrate body 10 is less than the height L′ of each of the first conductive bumps 11 with respect to the substrate body 10.
Therefore, it has become critical as to how to overcome the above-described drawbacks.